1. Field of the Invention
The present invention relates to a continuously variable transmission (CVT) for a vehicle, and a control device and method for the CVT.
2. Description of Related Art
Conventionally, electronically-controlled continuously variable transmissions (ECVTs) have been known. ECVTs can adjust the gear ratio irrespective of engine speed and are widely used in vehicles such as scooters.
In an ECVT, in general, feedback control of the gear ratio between an input shaft and an output shaft is performed to achieve a target gear ratio. However, unintentional vibration above and below a target gear ratio (“hunting”) may occur because of changes over time in characteristics of a slide mechanism of the ECVT, sensor output abnormalities, mismatches in control gain due to individual differences among ECVTs, and so forth. Such hunting in the gear ratio deteriorates drivability of the vehicle and places a large load on and deteriorates a gear ratio varying motor.
JP-B-Hei 8-23313 discloses a method to control a motor for driving an exhaust control valve, in which the motor is temporarily stopped when it has hunted a predetermined number of times. It is conceivable to apply this technique to the gear ratio varying motor of an ECVT.
However, the gear ratio may not be at the target gear ratio when the gear ratio varying motor of the ECVT is temporarily stopped after a predetermined number of huntings. That is, it may not be possible to achieve the target gear ratio. In addition, a load may be placed on the input and output shafts of the ECVT while the gear ratio control means is temporarily stopped, causing deviation from the target gear ratio. Thus, the technique of JP-B-Hei 8-23313 cannot suitably control an ECVT, even if hunting is suppressed to some degree.